Increase in price and the foreseeable shortage of energy have drawn the problem of effective structural insulation against a loss of heat into the foreground. In these considerations, the dynamic heat transmission through the shell of the building is dominant; for this reason the thermal optimization of the shell of the building required as a central focus insulation against loss of heat, primarily by the reduction of heat transmission through the structural parts of the shell of the building by means of thermal insulation.
The areal thermal transmittance is characterized by the K-Value and mathematically determined according to present practice from the mathematical values of the thermal conductivity of the building materials. These values were, in turn, determined by numerous measurements. Note the article "Transmission of Heat By Conduction and Convection" by W. H. Adams in pages 92 and 93 of the Mechanical Engineers Handbook, Mc Graw Hill book Co., 6th Edition (1958).
The determination of the K-value can be an actual measurement from the thermal transmission of the structure. In construction engineering, one speaks, therefore, of the K-value measurement, which means precisely this thermal transmission measurement.
Since the K-value quantifies the thermal insulation capabilities of the structure, it can be defined by certain standards. The planner can follow these standard requirements accordingly. Inaccuracies in construction, alterations of the plans, utilization of building materials of different quality, uncoordinated working stretches etc. can, however, give rise to thermal insulation abilities (K-values) different from the original plan. The necessity for monitoring the heat transmission coefficient is therefore obvious.
The monitoring of the heat transmission of a building is particularly problematical because the measurement has to take place under non-steady state conditions, that is, under constantly changing temperatures and air currents as well as under changing thermal radiation conditions. The measuring of the K-value under these non-steady conditions takes place, then, by way of the heat flow measurement, whereby there are three prerequisites, namely: a constant heat flow direction over a long period a sufficient integration period in order to attain representative average values, and continual control of the measuring.
These three prerequisites lead to difficulties that make a routine execution of such measurments impossible because:
the known methods of measurement make measuring possible only in the cold season;
the natural mean temperature difference has to lie above 10 K.;
outside constructions bathed in sunshine connot be measured; and
an almost steady room temperature is required and only very long measuring times permit the determination of representative results.